1. Field of the Invention
The present invention relates to an induction motor control apparatus, and in particular to a control apparatus which controls the power factor at which the motor operates, to provides a high efficiency of operation.
2. Description of the Related Art
A power factor control apparatus has been described in the prior art in U.S. Pat. No. 4,052,648 whereby the power factor of an induction motor is detected and the voltage applied to the motor is controlled accordingly, to thereby achieve an improvement in the operating efficiency of the induction motor by reducing the power consumed. With such an apparatus, the power factor of the induction motor is detected based upon a phase difference between voltage applied to the motor and the motor current. However, due to waveform distortion of the voltage and current of an induction motor, it is not possible to detect the power factor accurately by such a method. It has therefore been difficult to accurately control an induction motor to achieve a high level of operating efficiency. With that prior art control apparatus, the voltage applied to the induction motor is controlled such that the detected power factor is made to approach a preset value of power factor. If the preset value of power factor is made for example 80%, and the induction motor is operating under a condition of light load or no load, then it becomes necessary to excessively lower the voltage applied to the induction motor in order to make the detected value of power factor approach the preset power factor. Such an excessively low level of motor voltage results in stalling or cogging of the motor. To overcome that problem, it has been proposed to establish a cogging prevention voltage, separately from the preset power factor, and to compare the detected power factor with a cogging prevention voltage level, instead of with the preset value of power factor, to thereby prevent the occurrence of cogging or stalling when the induction motor is operating under a condition of no load or light load. However if that is done, the cogging prevention voltage cannot be set to a high value. Thus, it is not possible to accurately control the induction motor voltage when under low load or no load operation, so that highly efficient operation of the motor cannot be achieved. Moreover, if the induction motor is operated at an intermediate level of load, with a preset power factor of 80%, then since the preset value of power factor is excessively high with regard to that level of load, the voltage applied to the induction motor will be excessively low, and a high level of motor current, with the result that the power consumption of the motor will be increased. Thus in order to minimize the power consumption of the motor, it is necessary to manually adjust the preset power factor value of such a power factor control apparatus, in accordance with the load condition under which the induction motor is currently operating. Alternatively stated, if the preset power factor value is held fixed, with such an induction motor control apparatus, then is impossible to achieve efficient operation of the motor when changes occur in the motor load.
Another prior art induction motor control apparatus has been described in Japanese Patent Laid-open No. 64-50792, whereby efficient operation of an induction motor is to be achieved by varying the power factor based on the frequency of the power supplied to the motor, by using an inverter apparatus. With that apparatus, as shown in FIG. 3 of the specification of that invention, optimum power factor patterns for respectively different optimum power factor values at each of various frequencies are stored in a memory. The inverter operation frequency can be set, and the value of voltage applied to the motor can be set to obtain a fixed value of power factor corresponding to that frequency. However accurate voltage control for various different load conditions cannot be achieved.
Thus, with the first-mentioned prior art induction motor control apparatus, the accuracy of power factor detection is poor, due to waveform distortion of the induction motor voltage and current. With the second prior art induction motor control apparatus, with the frequency held fixed, the power factor will be held fixed even if variations in the motor load occur. Thus, it is not possible to achieve energy-saving operation for various different values of motor load.